Think: CrossFit hollow hold, but flipped (and not quite Superman). It's an insane core exercise, and while it might not look like you're doing anything, 30 seconds will feel like 30 years in this position. It's a doozy (apparently, an effective doozy).
Lay face down with your forehead to the ground, hips pressed down, arms extended overhead and reaching forward.
Scoop your belly button toward your spine, keeping your stomach from touching the ground (this is where the "hollow" comes in)
Keep the tops of your feet and hips pressing into the ground, and hold that belly scoop for 30 seconds.